1. Field of the Invention
The present invention relates generally to wave soldering control apparatus and, more particularly, to an apparatus for simultaneous measuring and checking several parameters of a solder contact, which could be provided by the solder wave when soldering a device like a printed circuit card.
2. Description of the Prior Art
Where it is necessary to provide soldering of a large number of electrical, electronic components, subsystems or systems such as on a printed circuit card, for example, it is not practical to execute this by a point-to-point manual soldering technique. Instead, such large number of connections are soldered together by an apparatus operating on what is referred to as a "wave" soldering principle. In a wave soldering machine, a printed circuit board to be soldered is moved by a conveyor through an enclosure on an inclined path past a fluxing station, a preheating station, and, at the end, a soldering station at which a wave of solder is caused to well upwardly and contact the various parts to be soldered. One of the most important parameters for insuring efficient and reliable soldering connections, is the dwell time of the devices to be soldered in the soldering wave.
In the prior art, one of the known techniques for measuring dwell time was by indirect means, in which a wave "footprint" measurement device was used and a plot was developed from empirical data. Specifically, a glass plate, having a set of grid lines of known dimensions is passed along the wave soldering machine conveyor and the contact of the solder wave is measured by the eye counting grid squares or lines of contact. Thus, it is a simple matter of calculating the dwell time by multiplying the thickness of the wave as measured on the plate by 60 seconds per minute and dividing by the conveyor speed in centimeters per minute, for example.
Although this technique is accurate, it leaves much to be desired in that for a number of different reasons the dwell time can change quickly. For example, there is an undesirable effect termed "drag-out", that may result from the wetting ability of the materials used, and the component lead configuration, in which the solder wave is pulled out of shape as the solder joints leave the wave. When this happens, the dwell time for certain joints will be significantly longer than for others. As the glass plate is not wetted by the solder, the known "footprint" technique does not indicate the presence of drag-out.
U.S. Pat. No. 5,023,848 provides a solder dwell time measurement means (timer) which operates automatically, does not rely upon human observational techniques and calculations, and can exhibit the presence of drag-out. The timer comprises a pair of spaced apart probes generally approximating the contact geometry of a conveyed object to be wave soldered. It is similarly conveyed past and in contact with the solder wave. A timing circuit interconnects with the probes and measures the dwell time of the pair of probes in contact with the solder wave. The probes' contacting parts may have shapes of linear edges or points.
The disadvantages of the described device are as follows. Despite the fact, that the dwell time is an important parameter of the wave soldering process, there are several other paramaters which must be defined and taken into account before starting the working process. Such a parameter, for example, is the depth of penetration of the solder wave into the contact orifices of the printed circuit. The other parameter is parallelity of the printed board when conveyed over the solder wave. This parameter has a strong influence on the quality of the soldered printed board.
It is therefore the object of the present invention to provide an ellaborated indicator of the solder wave process parameters, such parameters being the dwell time, the depth of penetration of the solder wave into the contact orifices of a board and the parallelity of the board on the conveyor.